Torelli loci, product cycles, and the homomorphism conjecture for $\mathcal{A}_g$

Abstract

The tautological $\mathbb{Q}$-subalgebra $\mathsf{R}^*(\mathcal{A}_g) \subset \mathsf{CH}^*(\mathcal{A}_g)$ of the Chow ring of the moduli space of principally polarized abelian varieties is generated by the Chern classes of the Hodge bundle. There is a canonical $\mathbb{Q}$-linear projection operator $\mathsf{taut}: \mathsf{CH}^*(\mathcal{A}_g) \rightarrow \mathsf{R}^*(\mathcal{A}_g).$ We present here new calculations of intersection products of the Torelli locus in $\mathcal{A}_g$ with the product loci $\mathcal{A}_{r}\times \mathcal{A}_{g-r} \rightarrow \mathcal{A}_g$ for $r\leq 3$. The results suggest that $\mathsf{taut}$ is a $\mathbb{Q}$-algebra homomorphism, at least for special cycles. We discuss a conjectural framework for this homomorphism property. Our calculations follow two independent approaches. The first is a direct study of the excess intersection geometry of the fiber product of the Torelli and product morphisms. The second recasts the geometry in terms of families Gromov-Witten classes, which are computed by a wall-crossing formula related to unramified maps. We define tautological projections of cycles on the fiber products $\mathcal X_g^s \to \mathcal A_g$ of the universal family. We compute these projections for a class of product cycles on $\mathcal X_g^s$ in terms of a determinant involving the universal theta divisors and Poincaré classes. Using Abel-Jacobi pullbacks of product cycles on $\mathcal X_g^s$ and their projections, we construct a new family of classes which we conjecture to lie in the Gorenstein kernels of the tautological rings $\mathsf{R}^*(\mathcal M^{\mathrm{ct}}_{g,n})$. In particular, we construct a nontrivial element of the Gorenstein kernel of $\mathsf{R}^5(\mathcal{M}_{5,2}^{\mathrm{ct}})$.

Figures (4)

• Figure 1: Star-shaped graph with partition labels on edges.
• Figure 2: An example of an unramified map from a nodal genus $2$ curve to a Fulton-MacPherson degeneration of an abelian surface.
• Figure 3: Graphs for $r=2$ and $g=4$, where $(1^2)$ denotes the partition $(1,1)$.
• Figure 4: Graphs for $r=2$ and $g=5$, where $(1^2)$ denotes the partition $(1,1)$.

Theorems & Definitions (58)

• Theorem 1: van der Geer vdg
• Definition 2: CMOP
• Definition 3: Iribar López Iribar
• Theorem 4
• Theorem 5: Excess contributions
• Theorem 6: Wall-crossing
• Theorem 7
• Conjecture 8: Strong form
• Conjecture 9: $\overline{\mathbb{Q}}$ form
• Theorem 11: Feusi-Iribar López-Nesterov aitordenisjeremy